This invention relates to impact detectors particularly, although not exclusively, for use with grain loss monitors for combine harvesters. Such impact detectors are used to give an indication to the machine operator of the amount of grain which is being lost either by way of being entrained in the straw which is discharged from the combine separating mechanism to the ground or by way of being discharged together with chaff and other impurities from the combine cleaning apparatus. Throughout this specification the reference to "grain" is intended to refer to that part of the crop which is threshed and separated from the discardable part of the crop which is referred to as "straw".
There are a number of problems associated with existing grain loss monitors and perhaps the most important problem is that there is not available a monitor which will indicate, in absolute terms, the amount of grain being lost during the harvesting process. However, the present invention nevertheless does not address this particular problem but rather the problems related to the variations in the sensitivity across the length of the detector surface of some known impact detectors on the one hand and to an insufficient detection rate capacity of other known impact detectors on the other hand.
One such impact detector manufactured by the Applicant is shown in EP-A-0.117.587 and is in the form of a plate of a synthetic plastics material which is set within a hollow metallic mount and is isolated therefrom on all sides. This sensing plate has dimensions in the range of e.g. 15 cm.times.7 cm and as such is rather small. Between the synthetic plastics plate and the base of the hollow mount, there is provided a damping material in the form of a sheet of foamed synthetics material (although other types of damping material may be employed). This damping sheet extends over the entire undersurface of the synthetic plastics detector plate. A crystal transducer is attached to the underside of the synthetics plate in the centre thereof.
The aforementioned damping sheet is intended to eliminate, to the extent possible, the transfer of machine vibrations from the metallic mount to the synthetic plastics material sensor plate and thus also to the transducer attached thereto. Problems which arise with such a detector include a reduction of the overall sensitivity thereof due to the foamed synthetics mounting material also dampening the vibrations within the sensor plate induced by the impacts to be detected. Furthermore, the sensitivity also widely varies across the surface of the sensor plate, which may be in part because the useful sensitive area is relatively small and is centred on the crystal transducer. These problems, in part, also are due to and are aggravated by the fact that the synthetic plastics material of the sensor plate has only a small vibration conductivity.
The foregoing means that, to date, quite often, a plurality of detectors have had to be employed across the width of the harvester at the point or points where grain loss is to be monitored. Typically, such points would be the end of the straw walkers and the end of the upper cleaning sieve in a conventional combine harvester.
In another known arrangement, the sensor plate is similar to the sensor plate described hereabove but with the difference being that this sensor plate (with the crystal transducer attached thereto) is mounted within a hollow metallic mount via foamed synthetic material that is provided only adjacent the circumferential edges of the sensor plate. This foamed synthetic material provided circumferentially at the edges of the sensor plate again is intended to isolate the sensor plate from the machine vibrations to the extent possible and accordingly is specifically selected for this purpose. The problems associated with this impact detector are of a similar nature as the problems described hereabove and associated with the impact detector of EP-A-0.117.587. In other words, the overall sensitivity is low and moreover the sensitivity widely varies across the surface of the detector plate.
In an attempt to overcome these problems the synthetic material sensor plate of the foregoing prior art impact detector has been replaced by a metallic sensor plate of substantially the same relatively small dimensions. Metal indeed has a much higher vibration conductivity. However, this leads to other problems in as much as any impact on the metallic sensor plate generates a signal which lasts a comparatively long time because it is transmitted along and across the plate and then is reflected back from the respective edges of the plate. Accordingly a "ringing" of the signals takes place which results in a saturation of the detector at a low detection rate. As a further result, this detector with a metallic sensor plate is unable to adequately "count" high signal rates. This "ringing" effect is not influenced, i.e. is not reduced to any substantial degree, by the foamed synthetic mounting material provided at the outer edges of the sensor plate for attaching the same to the mount. This foamed synthetic material has been selected for its characteristic of being able to isolate to a great extent the sensor plate from the machine vibrations.
In still another arrangement a single impact detector is provided which comprises a detector plate member or sensor plate which extends across substantially the full width of the machine and to which crystal transducers are attached at intervals of about 20 cm. This brings about still other problems which are associated with the fact that the absolute sensitivity of individual transducers may be widely different, so that the sensitivity across the total surface of the sensor plate also may be very irregular for this reason alone. Electronic conditioning of the individual transducers may overcome this problem; however, this leads to a very complex and expensive apparatus which equally is not acceptable. Hence, a simplier approach is applied by which the signals of all transducers are amplified proportionally.
In connection with the foregoing it also should be kept in mind that impact detectors used with grain loss monitors should be able to discriminate between signals generated by grain kernals and signals generated by pieces of straw. Grain kernals generate a signal with a somewhat larger amplitude than straw pieces in any given transducer. Accordingly, it is possible to discriminate between these signals generated by any given transducer. However, if output signals of transducers with different absolute sensitivity levels are applied to the same electronic circuitry, it may be that the signals generated by straw pieces in the more sensitive transducers may come close to the signals generated by grain kernals in the less sensitive transducers. Accordingly, a problem may arise in this area to the extent that it may no longer be possible to clearly distinguish between all the signals.
This problem is also aggravated by the varying sensitivity distribution across the surface of the sensor plate at varying distances from each individual transducer. As already mentioned, the sensitivity indeed is much higher in the immediate vicinity of the transducer than at a relatively short distance (e.g. 15 cm) from said transducer. It therefore may be necessary, for guaranteeing a reading for all grain kernal impacts irrespective of where the impacts take place relative to each individual transducer, to lower the sensitivity threshold. However, this also may result in the fact that any one of the more sensitive transducers also will record straw impacts for grain impacts.
The combination of all the foregoing results in the fact that the adjustment of the average sensitivity of a multi-transducer impact detector is very critical. In other words, any average sensitivity setting may at the same time be too high for some transducers whereby these transducers also count straw impacts and too low for some other transducers whereby these other transducers of the same impact detector are unable to detect the more "remotely" positioned grain impacts. This is of course unacceptable in as much as it greatly affects the accuracy of the grain loss indication.
In still another prior art arrangement, a single metallic sensor plate has been provided which equally extends across the full width of the combine harvester. Because of the high vibration conductivity of metal, the sensitive area around the transducers is greatly widened in such impact detectors. This may be even to the extent that no more than one transducer is needed for a full width detector plate. However, as already indicated, there again are other problems associated with this type of impact detectors as well. Indeed, the high vibration conductivity of metallic sensor plates leads to the disturbance of the original impact vibrations caused by echoes of these vibrations at the edges of the sensor plate. In other words, any impact vibration within the detector plate "rings" much too long whereby premature saturation is obtained as has already been described hereabove. Accordingly, such impact detectors may be used only at locations and under circumstances where only low level grain flow rates may be expected. In other words, such an impact detector may not be used underneath a threshing or separating concave of a combine harvester for providing an indication of the grain flow rate (as opposed to the grain loss level) at this location.